The present invention relates to solid, blocked polyisocyanate-based compositions which can be cured at temperatures of from about 80° C. to about 220° C. and are useful for the production of a powder-coated substrates, even heat sensitive substrates such as wood, sensitive metals or plastic.
Conventional powder coating systems for industrial applications are generally based on one of two different crosslinking chemistries. Acid functional resins are cured with epoxides or hydroxyalkylamides. Alcohol functional resins are usually cured with blocked isocyanates. Although there are many variations of these types of systems, they all have as a common characteristic the need for a temperature of at least 140° C. for effective curing. In many cases, even higher reaction temperatures (e.g., 160-200° C.) are used. These high curing temperatures limit usage of powder coatings to heat resistant substrates such as metal (e.g., metal cabinets, household appliances, bicycle frames, etc.)
Powder coatings have the advantages of extremely low emissions and virtually 100% utilization of the coating material when the overspray is recycled. Therefore, it would be advantageous to be able to apply powder coatings to heat sensitive substrates such as wood and plastic. Powder coatings which can be cured at lower temperatures would also dramatically reduce the energy cost and make these coatings even more economically attractive.
Adducts from isocyanates and dicarbonylmethanes are known for their low temperature reactivity cure. (See, for example, S. Petersen Ann 562, p. 205 (1949).) Among the commonly used dicarbonylmethanes are diethylmalonate, ethylacetylacetate and acetylacetone. U.S. Pat. No. 2,826,526, for example, teaches the use of acetylacetic acid esters as blocking agents for isocyanates in adhesives. GB 1442024 teaches the blocking of biuret-based polyisocyanates with diethylmalonate for liquid coatings.
Dicarbonylmethane-blocked isocyanates react with alcohols at 80-120° C., depending on the specific reactants used. In comparison to all other blocked isocyanates, these blocked isocyanates offer the advantage of the lowest curing temperatures achieved within this chemistry. (See D. A. Wicks, Z. W. Wicks Jr., “Multistep Chemistry in Thin Films: The Challenge of Blocked Isocyanates”, Prog. Org. Coat. 43 (2001) 131-140.)
Although dicarbonylmethane-blocked isocyanates are disclosed in many publications, to date these products have been used only as liquids in organic solvents for coatings and for adhesives. See, for example, DE 19 73 1392 which broadly discloses solid blocked norbornane diisocyanates and the use of diethyl malonate as a blocking agent. The only specific blocked diisocyanates taught in this disclosure are blocked with ε-caprolactam or triazole. No malonate-blocked materials are disclosed.
No dicarbonylmethane-blocked isocyanate products which satisfy the processing requirements of materials for powder coatings are known.
Any material used in the powder coatings must be a solid, grindable, friable material that remains a free flowing powder during storage. It must also be stable to blocking when formulated as a powder coating and should be processable in a standard powder coating process. A crosslinking agent which satisfies these requirements and which makes it possible to produce a powder coating that may be cured at temperatures below 150° C. would be commercially advantageous.